Crawler frame for construction machine

ABSTRACT

In a crawler frame in which legs in a center frame are formed from cast steel, a simple structure is achieved by reducing the number of components. To this end, the crawler frame, which has a center frame composed of a central frame section for supporting a swing bearing and legs located on the right and left sides of the central frame section; and track frames disposed on the sides of the distal ends of the legs, respectively, of the center frame, is designed such that each of the legs is bifurcated into front and rear leg sections and the front and rear leg sections are respectively formed from cast steel.

TECHNICAL FIELD

The present invention relates to a crawler frame for a constructionmachine such as a hydraulic excavator.

BACKGROUND ART

Generally, construction machines such as hydraulic excavators have acrawler unit (undercarriage) having a crawler frame as a main body; anupper structure mounted on the crawler unit so as to turn around freelyupon the crawler unit; and a work implement, cab, engine and otherswhich are mounted on the upper structure.

The crawler frame includes a center frame for supporting the upperstructure so as to turnable upon the center of the crawler frame andtrack frames coupled to the right and left sides of the center frame,extending in a longitudinal direction. The right and left track frameseach support an idler and a drive wheel at the front and rear endsthereof respectively. The center frame is composed of a central framesection for supporting a swing bearing and legs which are disposed onthe right and left sides of the central frame section for coupling thecentral frame section to each track frame. The whole center frame ismade from sheet metal and is substantially H-shaped or X-shaped in itsplan view. A known structure for such a center frame is such that inorder to sustain the load imposed on the swing bearing positioned at thecenter, a desired number of vertical wall members are joined by weldingbetween an upper face plate on which the swing bearing sits and a lowerface plate located under the upper face plate (See Japanese Patent KokaiPublications No. 8-72615, No. 11-93209 and No. 2000-230252).

This prior art center frame structure has however revealed the problemthat since the right and left track frames are coupled to the centralframe section by the legs which are formed by sheet metal work with useof steel plates, steel plates complicated in shape are involved and thenumber of parts is increased. As a result, welded places and thereforethe number of welding processes increase, requiring tremendousmanufacturing time and manufacturing cost.

In addition, since the upper faces of the legs made of steel plates areflat, the mud which has penetrated into the machine during operation andtraveling adheres to and deposits on the upper faces of the legs. Thismud penetrates into the swing bearing causing damage to it, or movesonto the top faces of the track frames, interfering with the rotation ofthe track carrier rollers and causing lopsided wear of the track carrierrollers.

The accumulated mud is removed by washing which, however, needs a lot ofwater for removal and many washing processes, resulting in increasedcleaning cost. In addition, a large amount of mud is left in the washingsite after washing a construction machine so that the liveries who rentconstruction machines such as hydraulic excavators are bothered by theproblem of mud disposal.

As an attempt to solve the above problem, there has been proposed acenter frame structure in which the legs are made of cast steel and thebase section of each leg is provided with a flange which is welded to aside face of a box-shaped central frame section. This arrangement raisesthe possibility that not only the number of parts but also the number ofwelding processes can be reduced, leading to a considerable reduction inthe number of processes and processing time. Further, by forming eachleg into a tubular shape and making the upper face of the leg convex incross-section, the problem of the adhesion/deposition of mud can besolved.

In this center frame structure including the cast legs, however, thebox-shaped central frame section is also formed from a sheet metal andthe legs are welded to the vertical walls made of a sheet metal. Forthis reason, this center frame leaves much to be desired in thestructure of the vertical walls that serve as reinforcement members forsustaining the load imposed on the swing bearing as well as in thenumber of parts, and therefore further simplification of the structureis required.

When producing cast legs for use in a middle-sized or largerconstruction machine, a large-sized molding box for steel castingbecomes necessary as such legs are large in outside dimension. Incasting by use of a large-sized molding box, a flow of hot water and gasventing are slow and a complicated casting method is involved, so thatcasting defects are likely to occur and manufacturing cost increases.

The present invention is directed to overcoming the foregoingshortcomings and a first object of the invention is therefore to providea crawler frame for a construction machine the structure of which issimplified by reducing the number of parts with legs of the center framebeing formed from cast steel. A second object of the invention is toprovide a crawler frame for a construction machine which has the meritof causing no problems in casting in addition to the merit of the firstobject.

DISCLOSURE OF THE INVENTION

The first object can be accomplished by a crawler frame for aconstruction machine according to the invention, the crawler framehaving:

a center frame composed of a central frame section for supporting aswing bearing and legs located on the right and left sides of thecentral frame section; and track frames disposed on the sides of thedistal ends of the legs, respectively, of the center frame,

wherein each of the legs is bifurcated into front and rear leg sectionsand formed from cast steel.

According to the invention, the legs for connecting the central framesection of the center frame to the track frames are made of cast steel,so that the number of parts can be reduced, complicated welding linesare not involved, and the number of welded places are reduced, whichleads to a significant reduction in the number of processes. Inaddition, the area formed from a steel plate or the like is reduced,thereby saving on material.

Since the legs made of cast steel can be designed to have an upper faceconvex in cross-section (e.g., the legs have a pentangularcross-section), mud is unlikely to deposit on the convex upper faces ofthe legs. Even if mud deposits on the convex upper faces of the legs, itwill be easily shaken off before fixing by vibration etc. that occursduring traveling, thanks to the effect of the fine-grained outer facesof the legs produced by the lost-wax process. For this reason, there isno mud accumulated on the convex upper faces of the legs. Even if thereis a chance that mud deposits on the upper faces, its amount isnegligible. As a result, the amount of water required for cleaning thehydraulic excavator as well as the number of cleaning process can bereduced, leading to a significant reduction in the cleaning cost.Further, the amount of mud left in the washing site after washing thehydraulic excavator is very small so that the problem of mud disposalimposed on the livery or the like can be alleviated.

Since the amount of mud accumulated on the legs and therefore the amountof mud accumulated on the track frames can be reduced, the track carrierrollers rotate smoothly, so that lopsided wear of the track carrierrollers can be avoided. Also, the reduction in the amount of mudaccumulated on the legs extremely reduces the amount of mud adhering tothe swing bearing and, in consequence, the swing bearing can hardly bedamaged.

By forming the legs from cast steel, the thickness of the legs can beeasily varied according to the load of the upper structure and,furthermore, the upper faces of the legs can be easily made in convexform which promotes falling off of depositing mud to prevent itsadhesion/accumulation without fail.

The second object can be accomplished by the invention in which each leghas a two-part structure in its base section. With this arrangement, theouter shape of the legs can be made small since the bifurcated legs areeach divided into the front leg section and the rear leg section andformed from cast steel.

Even when producing the legs for use in a relatively large (middle-sizedor larger) hydraulic excavator, a molding box, which is not so large,can be used like the prior art. As a result, a casting method can besimplified as well as a smooth flow of hot water and smooth gas ventingare ensured, which leads to prevention of casting defects andmanufacturing cost reduction.

Where the front and rear leg sections of each bifurcated leg areseparately formed by the lost-wax process, a relatively small moldingbox made of wax can be used like the prior art. Therefore, deformationof the molding box due to its own weight and the like can be avoided sothat high-accuracy, fine-grained legs having an outer face free fromsurface roughness can be obtained. The fine-grained legs provide goodsmoothness as well as improved appearance quality, so that when washingthe hydraulic excavator or during traveling, mud adhering to the legscan smoothly fall onto the ground.

In the invention, it is preferable that a base section of the front legsection be securely welded to a base section of the rear leg section anda base section of each leg at which the front and rear leg sections areintegrated with each other be securely welded to the central framesection. This makes it possible to securely integrate the front and rearleg sections with each other so that the rigidity of the legs can beimproved.

Preferably, the entire circumference of the base section of each leg iswelded to an upper face plate, a lower face plate, a front face plateand a rear face plate which constitute the central frame section. Withthis arrangement, the structure of the legs can be made stronger so thatthe load of the upper structure sustained by the central frame sectioncan be transmitted to the legs without fail and then evenly transmittedto the track frames through the legs.

In the invention, it is preferable that vertical walls formed from caststeel be provided for the front and rear leg sections so as to beintegral with their base sections respectively. With this arrangement,the load imposed on the swing bearing can be sustained by the verticalwalls of the legs so that it becomes unnecessary to provide verticalwalls for the central frame section at the positions where the legs arejoined to the central frame section. As a result, the number of partscan be further reduced and the desired strength can be achieved with asimple structure.

Preferably, the vertical walls of the legs are located substantiallyimmediately under a circular mount for supporting the swing bearing.With this arrangement, the load of the upper structure imposed on theswing bearing can be directly sustained by the vertical walls of thelegs, so that the most rational structure for supporting the load of theupper structure can be achieved.

Preferably, the vertical walls respectively have a hole through which ahydraulic oil pipe is passed and a lip defining this hole is thickened.This not only facilitates laying of a hydraulic oil pipe which extendsfrom a hydraulic pump disposed in the upper structure to a hydraulicmotor disposed on the track frame side, but also contributes to areduction in the weight of the legs. In addition, the lip of the holecan be reinforced and rounded by thickening, so that there is no need toprovide a grommet such as used for a piping hole of the vertical wallmade of a sheet metal.

Preferably, the upper and lower face plates of the central frame sectionare joined to each leg by J groove welds and the surfaces of the upperand lower face plates are flush with the upper and lower faces,respectively, of the leg. With this arrangement, the height of the legscan be increased thereby achieving improved rigidity and allowance foradjustment can be obtained by the J groove welds so that tacking andalignment become easy and stress concentration is unlikely to occur.

According to another embodiment of the invention, the central framesection has right and left side supporting plates and the base sectionsof the legs are inserted into and securely welded to the central framesection so as to face the side supporting plates respectively. It ispreferable that the side supporting plates be located substantiallyimmediately under the circular mount for supporting the swing bearing.With this arrangement, the load of the upper structure imposed on theswing bearing can be directly sustained by the side supporting plates sothat the load of the upper structure can be steadily borne.

In this embodiment, the side supporting plates may be respectivelyprovided with a hole through which a hydraulic oil pipe is passed and agrommet may be fitted on a lip defining this hole. This facilitateslaying of the hydraulic oil pipe which extends from the hydraulic pumpdisposed in the upper structure to the hydraulic motor located on thetrack frame side. Further, the provision of the grommet on the lip ofthe hole has the effect of preventing damage to the hydraulic oil pipepassing through the hole.

According to still another embodiment of the invention, a vertical platesection is formed at the rear end of the base section of the front legsection and at the front end of the base section of the rear leg sectionand the base sections of the front and rear leg sections aresubstantially rectangular in cross-section. In this embodiment, the loadof the upper structure imposed on the swing bearing can be sustained bythe vertical plate sections formed in the front and rear leg sections,so that the load of the upper structure can be steadily borne.Additionally, the side of each leg facing the central frame section canbe opened, which facilitates laying of the hydraulic oil pipe.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a general perspective view of a crawler frame according to afirst embodiment of the invention.

FIG. 2 is a plan view of the crawler frame according to the firstembodiment.

FIG. 3 is a front view of the crawler frame according to the firstembodiment.

FIG. 4 is a side view of the crawler frame according to the firstembodiment.

FIG. 5 is a perspective view of a leg of the crawler frame according tothe first embodiment when viewed from underneath.

FIG. 6 is an exploded perspective view of a central frame sectionaccording to the first embodiment.

FIG. 7 is a view showing a cross-section of a portion in theneighborhood of a vertical wall formed in the leg of the firstembodiment.

FIG. 8 is a perspective view of a leg of a crawler frame according to asecond embodiment when viewed from underneath.

FIG. 9 is an exploded perspective view of a central frame sectionaccording to the second embodiment.

FIG. 10 is a cross-sectional view showing a joint structure which joinsa front leg section to a rear leg section according to the secondembodiment.

FIG. 11 is a cross-sectional view showing a joint structure which joinslegs to upper and lower face plates according to the second embodiment.

FIG. 12 is an exploded perspective view of a central frame sectionaccording to a third embodiment of the invention.

FIG. 13 is a cross-sectional view showing a joint structure which joinsa front leg section to a rear leg section according to a fourthembodiment of the invention.

FIG. 14 is a partly perspective view of a crawler frame according to afifth embodiment of the invention.

FIG. 15 is a plan view of the crawler frame according to the fifthembodiment.

FIG. 16 is a partly perspective view of a crawler frame according to asixth embodiment of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring now to the accompanying drawings, a crawler frame for aconstruction machine will be concretely described according toembodiments of the invention.

First Embodiment

FIG. 1 is a general perspective view of a crawler frame applied to ahydraulic excavator according to a first embodiment of the invention.FIGS. 2, 3 and 4 are a plan view, front view and side view,respectively, of the crawler frame according to the first embodiment.FIG. 5 is a perspective view of a leg when viewed from underneath.

A crawler frame 1 of this embodiment is constituted by a center frame 2and track frames 3A, 3B disposed on the right and left sides of thecenter frame 2 so as to extend in the back and forth direction of thevehicle body. Each track frame 3A (3B) includes a supporting frame 4located at the center and having a portal cross-section; and an idlersupporter 7 and drive wheel supporter 8 which are supported at the frontand rear ends of the supporting frame 4 through plate members 5, 6respectively. An idler and a drive wheel (both are not shown) aresupported by the idler supporter 7 and the drive wheel supporter 8,respectively.

The center frame 2 is composed of a central frame section 9 and legs 10,11 disposed on the right and left sides of the central frame section 9.The distal ends of the legs 10, 11 are joined to the sides of thesupporting frames 4 of the track frames 3A, 3B, respectively.

The central frame section 9 is formed from a material such as a steelplate and has, as shown in FIG. 6, (i) a substantially semi-oval upperface plate 12 having a hole 12 a at the center thereof through which aswivel joint, a pipe, etc. are passed; (ii) a lower face plate 13 havinga front wall (front face plate) 13 a which is formed by bending thefront edge of the substantially semi-oval lower face plate 13 so as torise vertically; (iii) a rear face plate 14 composed of three faceportions formed by bending the rear face plate 14 with vertical bendinglines at the desired right and left positions so as to enclose the rearparts of the upper face plate 12 and lower face plate 13. Herein, theheight of the front wall 13 a is equal to the height of the rear faceplate 14.

The central frame section 9 is formed in the following way: The rearface plate 14 is placed on the upper surface of the lower face plate 13at a slightly more inward position from the oval-shaped rear curved edgeof the lower face plate 13. The rear face plate 14 is positioned suchthat the middle face of the rear face plate 14 is located at in themiddle of the oval-shaped rear curved edge part of the lower face plate13 and, then, welded to the lower face plate 13 so as to stand uprightthereon. Subsequently, the lower surface of the upper face plate 12 isjoined to the upper ends of the front wall 13 a and the rear face plate14 by welding. In this way, the central frame section 9 is assembled inthe form of a box having open sides. Thereafter, a circular mount 15 forsupporting a swing bearing is placed on and secured to the upper surfaceof the upper face plate 12.

Although it has been described that the front wall 13 a is formedintegrally with the lower face plate 13 by bending the front side ofspecified width of the lower face plate 13 so as to rise vertically, thelong rectangular front wall and the lower face plate which issubstantially semi-oval on the whole may be separately formed byseparately cutting plates having specified sizes out of a sheetmaterial.

The legs 10, 11 disposed on the right and left sides of the centralframe section 9 are respectively bifurcated so as to have asubstantially V-shape in plan. In the plan view of the legs 10, 11, theyhave a substantially X shape when viewed as a whole. By virtue of this,the load of the upper structure imposed on the central frame section 9is dispersedly distributed to four leg sections so that the load can beeffectively sustained. The left leg 10 and the right leg 11 areaxisymmetric and have the same structure, and therefore the structureetc. of the left leg 10 will be explained as a representative in thefollowing description.

The left leg 10 is bifurcated into two parts, that is, a front legsection 10A and a rear leg section 10B, and has a two-part structure atits root section (base section). These front and rear leg sections 10A,10B are integrally formed from cast steel and have a tubular shape inwhich the upper face is convex and the middle part is substantiallypentangular in cross-section. The front leg section 10A and the rear legsection 10B are fitted each other and integrated by welding at a joint10 a in the base section of the leg 10. Herein, the front leg section10A and the rear leg section 10B are joined to each other by the samejoining method (See FIG. 10) as in the second embodiment describedlater.

As shown in FIG. 5, vertical walls 10 b are integrated with the basesection of the leg 10 and have circular holes (cast holes) 10 c, 10 dwhich communicate with the inner space of the leg sections 10A, 10Brespectively. The proximal end face of the leg 10, that is, the surfacesof the vertical walls 10 b describes a circular arc having substantiallythe same curvature as that of the inner circumferential face of thecircular mount 15. When the leg 10 is joined to the central framesection 9, the vertical walls 10 b are positioned immediately below thearced portion of the circular mount 15.

In the base section of the leg 10, stepped sections 10 e are formed sothat the base section is lowered. This stepped sections 10 e are curvedin line with the side edges of the upper face plate 12 and lower faceplate 13 of the central frame section 9 in plan. The side edges of theupper and lower face plates 12, 13 are brought into contact with thestepped sections 10 e and welded to the leg 10. A front face portion 10f of the leg 10 positioned closer to the proximal end of the leg 10 thanthe stepped sections 10 e is welded to the inner side face of the frontwall 13 a of the lower face plate 13 in the central frame section 9. Arear face portion 10 g of the leg 10 is welded to the inner side face ofthe rear face plate 14 in the central frame section 9.

Provided at the distal end of the front leg section 10A is a jointflange section 10 h which is brought into contact with and welded to theinner wall face of the supporting frame 4 of the track frame 3A.Provided at the distal end of the rear leg section 10B are a jointflange section 10 i and a joint flange section 10 j. The joint flangesection 10 i is brought into contact with and welded to the inner wallface of the supporting frame 4 of the track frame 3A. The joint flangesection 10 j is in the form of Japanese letter

and continuously extends from the joint flange section 10 i. The jointsection 10 j is welded to the plate member 6 secured to the rear end ofthe supporting frame 4.

Reference is made to FIG. 7 to describe the joint structure which joinsthe upper face plate 12 and the lower face plate 13 to the leg 10 aswell as the structure of the vertical walls 10 b in detail.

As shown in FIG. 7, the stepped sections 10 e are formed at the jointswhich join the upper face plate 12 and the lower face plate 13 to theleg 10. A J groove weld 16 is formed between the upper face of the upperstepped section 10 e and the end face of the upper face plate 12 andbetween the lower face of the lower stepped section 10 e and the endface of the lower face plate 13. Thus, the surfaces of the upper faceplate 12 and the lower face plate 13 are made flush with the upper andlower faces of the leg 10 respectively by welding the J grooves 16.

The provision of the J groove welds 16 has such a merit that the heightof the leg 10 can be increased, thereby achieving improved rigiditycompared to the conventional fillet welding. Additionally, allowance foradjustment can be obtained by the J groove welds 16, so that tackwelding and alignment (adsorption of the permissible deviations of thesizes of the leg 10 and the central frame section 9) can be easily doneand stress concentration is unlikely to occur at the weld joints.

The holes (cast holes) 10 c, 10 d provided for the vertical walls 10 bof the leg 10 are formed such that their peripheries (lips) arereinforced by thickening (rimming) and the thickened parts are rounded.Passing through either of the holes 10 c, 10 d is a hydraulic oil hoseor the like which extends from a hydraulic pump disposed in the upperstructure to a hydraulic motor disposed on the side of the track frame3A. This not only facilitates laying of the hydraulic oil pipe, but alsoreduces the weight of the leg 10. Since the thick lips of the holes 10c, 10 d are rounded, the hydraulic oil pipe 17 will not get scratchedeven if it bumps against the thick lip and therefore there is no need toattach a grommet to the lips of the holes unlike the case of theconventional vertical walls made of a sheet metal. Moreover, these holes10 c, 10 d are formed not by laser beam machining like the verticalwalls made of a sheet metal but by die cutting, so that they can beeasily formed.

According to the invention, there is no need to provide the centralframe section 9 with vertical walls which are conventionally joined tothe legs 10, 11 and therefore the number of parts can be reduced.Additionally, since the vertical walls 10 b provided for the legs 10, 11are positioned immediately below the arced portions of the circularmount 15, the load of the upper structure imposed on the swing bearingcan be directly sustained by the vertical walls 10 b, which isbeneficial in view of strength.

The center frame 2 of the first embodiment composed of the central framesection 9 and the bifurcated legs 10, 11 substantially takes the form ofX in plan when viewed as a whole, so that the load of the upperstructure imposed on the circular mount 15 disposed on the upper faceplate 12 of the central frame section 9 is dispersedly distributed tothe four leg sections of the bifurcated legs 10, 11 and the load is thuseffectively sustained. A relatively large left aperture section 65 isdefined by the left bifurcated leg 10 and the left track frame 3A whichare integral with each other, whereas a relatively large right aperturesection 66 is defined by the right bifurcated leg 11 and the right tackframe 3B which are integral with each other (See FIG. 2). Duringexcavating operation and traveling/turning movement of the constructionmachine such as a hydraulic excavator, it frequently happens that flyingmud penetrates into the excavator and part of it adheres to the topfaces of the legs 10, 11. In such a situation, the adhering mud easilydrops onto the ground through the relatively large right hole 65 andleft hole 66 owing to vibration or the like during excavating operationand traveling/turning movement. While the first embodiment has beendescribed with the center frame 2 which has a substantially X shape inplan when viewed as a whole, the center frame 2 may have a substantiallyH shape in plan when viewed as a whole.

The entire peripheries of the upper/lower faces, front side and rearside of the base section of each integrated bifurcated leg 10 (11) aresecurely welded to the side edges etc. of the constituents of thecentral frame section 9. Therefore, not only can strong fixation beestablished but also the load of the upper structure can be distributedto and steadily sustained by the four leg sections connected to theright and left track frames 3A, 3B.

The legs 10, 11 have a convex upper face and substantially pentangularcross-section and are formed from cast steel, so that even if mudpenetrates into the upper faces of the legs 10, 11 during operation, iteasily drops without adhering to and depositing on the upper faces.Therefore, the mud does not get into the swing bearing causing damage toit, nor does it move to the upper faces of the track frames 3A, 3Binterfering with the rotation of the track carrier rollers. The legs 10,11 are accordingly excellent in the ability of blowing off mud. Thanksto this, the amount of water used for washing the hydraulic excavator aswell as the number of washing processes can be reduced, leading to asignificant reduction in the cleaning cost. In addition, the amount ofmud left in the washing site after washing the hydraulic excavator canbe markedly reduced so that the problem of mud disposal suffered by theliveries can be alleviated.

It is generally difficult to manufacture a whole bifurcated leg for usein a middle-sized or larger construction machine by casting because theleg is large in size and its structure is more or less complicated. Inthe first embodiment, the bifurcated legs 10, 11 can be divided into twosmall parts, i.e., the front leg section and rear leg section and thesetwo parts are independently manufactured by casting. Therefore, evenwhen a relatively large leg is produced, each part to be cast is not solarge. As a result, a relatively small molding box can be used like theprior art. This enables defect-free casting with a smooth flow of hotwater and smooth gas venting, leading to a reduction in the productioncost.

Where the front leg section 10A and the rear leg section 10B areseparately manufactured by the lost-wax process for example, a moldingbox formed from wax is used which is not extremely large and istherefore unsusceptible to deformation caused by its own weight or thelike so that production of high-precision legs is enabled. Moreover, byuse of the lost-wax process, the outer surfaces of the legs can be freefrom roughness, which leads to not only improved appearance quality butalso good smoothness. As a result, even if mud adheres to the legs, itwill fall on the ground after a while so that no or little mud isaccumulated on the legs.

In addition, since the legs 10, 11 are made of cast steel in tubularform, their thickness can be easily varied according to the load to beimposed thereon, thereby making the internal stress (bending stress,shearing stress) of the legs 10, 11 almost uniform. For instance, thelegs 10, 11 can be formed such that the area close to the track frame 3A(3B) and subjected to high internal stress is thickened and thicknessis, then, gradually reduced toward the side close to the central framesection 9. With this arrangement, the legs 10, 11 can be madelightweight compared to the case of legs having uniform thickness whichis determined in compliance with the highest stress imposed thereon likethe conventional legs made of sheet metal.

While the cross-section of the legs is substantially pentangular in thefirst embodiment, it may be substantially triangular, quadrangular orhexagonal with a bulged top face.

Second Embodiment

FIG. 8 shows a perspective view of a leg according to a secondembodiment. FIG. 9 shows an exploded perspective view of a central framesection according to the second embodiment. The whole structure of thecrawler frame of the second embodiment is basically the same as that ofthe first embodiment shown in FIGS. 1 to 4 and therefore the figurescorresponding to FIGS. 1 to 4 and a detailed explanation of the partssimilar to those of the first embodiment are skipped herein.

In the second embodiment, a central frame section 19 is formed from asteel plate or the like as shown in FIG. 9 and constituted by (i) asubstantially semi-oval upper face plate 21 having a hole 21 a at thecenter thereof through which a swivel joint, pipes and others arepassed; (ii) a lower face plate 23 having a front wall (front faceplate) 23 a which is formed by bending the front edge of thesubstantially semi-oval lower face plate 23; (iii) a rear face plate 27composed of three face portions formed by bending the rear face plate 27with vertical bending lines at desired right and left positions so as toenclose the rear parts of the upper face plate 21 and lower face plate23; (iv) a side supporting plate 24L located on the left side; and (v) aside supporting plate 24R located on the right side.

The upper face plate 21 has a uniform lateral width in a front portionhaving a specified length W. A semi-oval flange portion continues fromthe front portion having the length W. The lower face plate 23 has auniform lateral width in a front portion which has the specified lengthW and extends from the front wall 23 a. Extending backwardly from thefront portion of the lower face plate 23 is a semi-oval flange portion.

Each side supporting plate 24L (24R) is constituted by a face plateportion 24Lb (24Rb) and an arced face plate portion 24Lc (24Rc). Theface plate portion 24Lb (24Rb) has length substantially equal to thelength W and is parallel with the inner wall of the track frame 3A (3B).The arced face plate portion 24Lc (24Rc) is outwardly bulged, extendingfrom the face plate portion 24Lb (24Rb) along the inner circumferentialwall of the circular mount 15. The side supporting plates 24L, 24R areprovided with insertion holes 24La, 24Ra respectively through which ahydraulic oil pipe or the like is passed.

In the rear face plate 27, the central face portion is wider than theright and left face portions. By properly adjusting the width of thecentral face portion and others, the side edges of the right and leftface portions are firmly joined to the rear faces of the rear legsections of the right and left bifurcated legs respectively (describedlater).

Next, a process for forming the box-shaped central frame section 19 willbe described.

First, the left and right side supporting plates 24L, 24R are placed onthe upper surface of the lower face plate 23 at positions which are aspecified distance inwardly away from the edge of the front portionhaving the specified length W and uniform lateral width and from theedge of the semi-oval flange portion backwardly extending from the frontportion, such that the arced face plate portions 24Lc, 24Rc of the leftand right side supporting plates 24L, 24R are located immediately underthe inner circumferential wall of the circular mount 15. Then, the leftand right side supporting plates 24L, 24R are welded to the lower faceplate 23 so as to stand upright on the lower face plate 23. The frontedges of the face plate portions 24Lb, 24Rb which are parallel with eachother are welded to the rear face of the front wall 23 a of the lowerface plate 23. The rear face plate 27 is placed on the upper surface ofthe lower face plate 23 such that the central face portion of the rearface plate 27 is located just in the middle of the rear curved edge ofthe lower face plate 23. Then, the rear face plate 27 is welded to thelower face plate 23 so as to stand upright on the upper surface of thelower face plate 23 at a slightly more inward position from the rearcurved edge of the lower face plate 23. Thereafter, the lower face ofthe upper face plate 21 is welded to the upper end of the front wall 23a, the upper ends of the left and right side supporting plates 24L, 24Rand the upper end of the rear face plate 27 such that the profile of theupper face plate 21 coincides with the profile of the lower face plate23. Thus, the box-shaped central frame section 19 is formed, the bottomof which is constituted by the lower face plate 23, the side wall ofwhich is constituted by the front face plate 23 a, the right and leftside supporting plates 24L, 24R and the rear face plate 27, and the topof which is constituted by the upper face plate 21 having the hole 21 aat the center thereof.

As shown in FIG. 8, bifurcated legs 20 disposed on the right and leftsides of the central frame section 19 are each divided into two parts,i.e., a front leg section 20A and a rear leg section 20B at a rootportion (base section) similarly to the first embodiment (although FIG.8 shows the left leg 20 alone, the same is applied to the right leg).The front and rear leg sections 20A, 20B are made of cast steel andformed in the shape of an integral tube having a convex top face and amiddle portion of substantially pentangular cross-section. Each of thefront and rear leg sections 20A, 20B gradually diagonally inclines fromthe central frame section 19 to which its base section is secured towardthe track frame 3A (3B). Each leg sections 20A (20B) extends, describingan arc and its distal end is welded to the inner wall of the track frame3A (3B).

In the base section of the leg 20, stepped sections 20 e are formed suchthat the base section is lowered. This stepped sections 20 e are curvedin a plan view in line with the side edges of the upper face plate 21and lower face plate 23 of the central frame section 19.

For attaching the right and left legs 20 to the right and left sides ofthe central frame section 19, the following procedure is taken. First,the end edges of the base sections of the legs 20 are inserted so as toface the right and left side supporting plates 24L, 24R of the centralframe section 19, respectively. Then, the side edges of the upper andlower face plates 21, 23 are brought into contact with the steppedsections 20 e of the legs 20 respectively to weld the upper and lowerface plates 21, 23 to the legs 20. A front face portion 20 f of each leg20 is welded to the inner side face of the front wall 23 a of the lowerface plate 23 of the central frame section 19, whereas a rear faceportion 20 g of the leg 20 is welded to the inner side face of the rearface plate 27 of the central frame section 19.

Provided at the distal end of the front leg section 20A is a jointflange section 20 h which is in contact with and welded to the innerwall face of the supporting frame 4 of the track frame 3A. Provided atthe distal end of the rear leg section 20B are a joint flange section 20i and a joint section 20 j in the form of Japanese letter

. The joint flange section 20 i is in contact with and welded to theinner wall face of the supporting frame 4 of the track frame 3A. Thejoint section 20 j continuously extends from the joint flange section 20i and is welded to the plate member 6 secured to the rear end of thesupporting frame 4.

Now, reference is made to FIGS. 10, 11 to describe the joint structurewhich joins the front leg section 20A to the rear leg section 20B andthe joint structure which joins the leg 20 to the upper face plate 21and the lower face plate 23.

As shown in FIG. 10, in the joint which joins the front leg section 20Ato the rear leg section 20B, stepped sections 20 a are formed on theedges of the rear leg section 20B. A J groove 35 is formed between theupper face of the upper stepped section 20 a and the end face of thefront leg section 20A confronting the upper face. Another J groove 35 isformed between the lower face of the lower stepped section 20 a and theend face of the front leg section 20A confronting the lower face. Byintegrating the front leg section 20A with the rear leg section 20Bthrough welding of the J grooves 35, the surface of the front legsection 20A is made flush with the surface of the rear leg section 20Bso that the quality of the appearance of the leg 20 can be improved.

As shown in FIG. 11, the stepped sections 20 e of the base section ofthe leg 20 formed by integrating the front leg section 20A with the rearleg section 20B are inserted between the side edges of the upper andlower face plates 21, 23, whereas a J groove weld 35 is formed betweenthe upper face of the upper stepped section 20 e and the confronting endface of the upper face plate 21 and another J groove weld 35 between thelower face of the lower stepped section 20 e and the confronting endface of the lower face plate 23, whereby the bifurcated leg 20 can besecured to the central frame section 19. At that time, the surfaces ofthe upper and lower face plates 21, 23 are made flush with the upperface and lower face of the base section of the leg 20 respectively,thereby achieving improved appearance quality.

The side supporting plates 24L, 24R are joined by welds 36 to the upperand lower face plates 21, 23 at slightly more inward proper positionsfrom their side edges such that the side supporting plates 24L, 24Rstand upright. These side supporting plates 24L, 24R are provided withthe insertion holes 24La, 24Ra through which the hydraulic oil pipe 17or the like is passed. The insertion hole 24La (24Ra) is formed in thearced face plate portion 24Lc (24Rc), being located on the side of therear leg section 20B in order that it is communicated with the open endof the tubular rear leg section 20B. Herein, the arced face plateportions 24Lc, 24Rc are curved so as to extend along the innercircumferential wall of the circular mount 15. Although the sidesupporting plate 24L (24R) has only one insertion hole 24La (24Ra) inthis embodiment, another insertion hole 24La (24Ra) may be provided ifnecessary.

Grommets 24Lg, 24Rg are fitted on the peripheries (lips) of theinsertion holes 24La, 24Ra, thereby preventing the hydraulic oil pipe 17being hurt by the edges of the insertion holes 24La, 24Ra.

For assembling the central frame section 19, each side supporting plate24L (24R) is brought into contact with the upper face of the lower faceplate 23 at slightly more inward proper position from the left (right)side edge of the lower face plate 23 and joined to the lower face plate23 by forming the welds 36 from inside and outside the side supportingplate 24L (24R). Similarly, the rear face plate 27 is joined to theupper face of the lower face plate 23 at a slightly more inward properposition from the rear curved edge of the lower face plate 23, and thejoint is welded from inside and outside the rear face plate 27. Then,the upper face plate 21 is placed on the upper end of the front faceplate 23 a, the upper ends of the left and right side supporting plates24L, 24R and the upper end of the rear face plate 27 such that theprofile of the upper face plate 21 coincides with the profile of thelower face plate 23. Thereafter, the upper end of the front wall 23 a,the upper ends of the left and right side supporting plates 24L, 24R andthe upper end of the rear face plate 27 are joined to the lower face ofthe upper face plate 21 and these joints are securely welded from insidethrough the hole 21 a. Subsequently, each part is welded from outside,thereby forming the central frame section 19.

Third Embodiment

FIG. 12 shows an exploded perspective view of a central frame sectionaccording to a third embodiment of the invention. The third embodimentdiffers from the second embodiment in the shape of the central framesection. Except this, the third embodiment is the same as the secondembodiment.

In the second embodiment shown in FIG. 9, each side supporting plate 24L(24R) of the central frame section 19 is constituted by the flat faceplate portion 24Lb (24Rb) having the specified length W and the arcedface plate portion 24Lc (24Rc) extending from the portion 24Lb (24Rb).On the other hand, a central frame section 70 according to the thirdembodiment includes left and right side supporting plates 74L, 74R. Eachside supporting plate 74L (74R) includes a flat face plate portion 74Lb(74Rb) having a specified length W which is parallel with the inner wallof the track frame 3A (3B) and a flat face plate portion 74Lc (74Rc)which extends from the portion 74Lb (74Rb), being slightly tapered downbackward. Since it is more or less difficult to form the face plateportions 24Lc, 24Rc of the left and right side supporting plates 24L,24R into the shape of an arc which fits the inner circumferential wallof the circular mount 15 as shown in FIG. 9, the face plate portions74Lc, 74Rc of the left and right side supporting plates 74L, 74R aremade in linear flat form as the second best way in the third embodiment.Like the second embodiment, the side supporting plates 74L, 74R have theinsertion holes 74La, 74Ra, respectively, through which the hydraulicoil pipe or the like is passed.

The method of forming the box-like central frame section of the thirdembodiment is the same as that of the second embodiment except that theleft and right side supporting plates 74L, 74R are joined to the upperface of a lower face plate 73 and to the lower face of an upper faceplate 71 at more inward positions compared to the second embodiment.

Generally, the best application for the central frame section 70 of thethird embodiment is the case where the base sections of the right andleft legs are not curved in the shape of an arc which fits the innercircumference of the circular mount 15 but are formed into a linearshape. However, it may be applied to the case the base sections of thelegs are relatively gently curved.

Fourth Embodiment

FIG. 13 is a cross-sectional view showing a joint structure which joinsa front leg section to a rear leg section according to a fourthembodiment of the invention. While the second embodiment is designedsuch that the stepped sections 20 a are formed on the end edges of thefront leg section 20A and the J groove welds 35 are formed in thestepped sections 20 a as shown in FIG. 10, the fourth embodiment isdesigned such that the rear end edges of the front leg section 20A arereduced in thickness toward the rearmost part so that the rear end edgesare partly cut away in section whereas the front end edges of the rearleg section 20B are reduced in thickness toward the foremost part insection. The foremost parts of the rear end edges of the front legsection 20A and the foremost parts of the front end edges of the rearleg section 20B are brought into contact with each other, so thatV-shaped grooves 39 are formed at butt joints 20 b. A backing 41 is heldtightly against each V-shaped groove 39 beforehand for welding.

With the joint structure of the fourth embodiment, the joints betweenthe front leg section and the rear leg section can be firmly welded.

Fifth Embodiment

FIG. 14 is a partly perspective view of a crawler frame according to afifth embodiment of the invention. FIG. 15 is a plan view of the crawlerframe of the fifth embodiment.

In the fifth embodiment, bifurcated legs are each composed of two parts,i.e., a front leg section 57A and a rear leg section 57B like theforegoing embodiments. These leg sections 57A, 57B are respectivelyformed from cast steel and then united.

The front and rear leg sections 57A, 57B respectively have a basesection 57 c of substantially rectangular cross-section. A verticalplate section 57 a is formed at the rear end of the base section 57 c ofthe front leg section 57A, whereas a vertical plate section 57 b isformed at the front end of the base section 57 c of the rear leg section57B.

The vertical distance between the upper and lower faces of the rear endof the base section 57 c of the front leg section 57A is shorter thanthe vertical distance between the upper and lower faces of the front endof the base section 57 c of the rear leg section 57B. The rear end ofthe base section 57 c of the front leg section 57A is fitted in thefront end of the base section 57 c of the rear leg section 57B such thatthe vertical plate section 57 a of the front leg section 57A confrontsthe vertical plate section 57 b of the rear leg section 57B. It shouldbe noted that the vertical plate section 57 b of the rear leg section 57is set back slightly backward (inward) in order that the vertical platesection 57 a of the front leg section 57A can be inserted and fit in thefront end of the base section 57 c of the rear leg section 57B.

In the fifth embodiment, the vertical plate section 57 a of the frontleg section 57A is inserted and fitted in the front end of the basesection 57 c of the rear leg section 57B, thereby forming a J-shapedgroove 59 at a joint 57 d between the upper face of the rear end of thebase section 57 c of the front leg section 57A and the lower face of thefront end of the base section 57 c of the rear leg section 57B and at ajoint 57 d between the lower face of the rear end of the base section 57c of the front leg section 57A and the upper face of the front end ofthe base section 57 c of the rear leg section 57B. By forming a weld ineach J groove 59, the front leg section 57A and the rear leg section 57Bare integrated into the bifurcated leg 57.

The base sections 57 c of the bifurcated legs 57 thus integrated, thebase sections having a substantially rectangular cross-section, areinserted into the right and left side faces of the central framesection, respectively. By virtue of a vertical plate section which has astepped section 57 g and constitutes the front face of the base sectionof the front leg section 57A; a vertical plate section which has astepped section 57 m and constitutes the rear face of the base sectionof the rear leg section 57B; the vertical plate section 57 a located atthe rear end of the base section of the front leg section 57A; and thevertical plate section 57 b located at a slightly backward (inward)position from the front edge of the base section of the rear leg section57B, the load imposed on the circular mount 15 mounted on the top of thecentral frame section can be steadily sustained.

In cases where the vertical plate sections having the stepped sections57 g, 57 m are relatively thick, only the vertical plate section 57 amay be provided while the vertical plate section 57 b being omitted.

The upper and lower faces of the base section of each of the front andrear leg sections 57A, 57B are respectively provided with a steppedsection 57 e so that the upper and lower faces of the base section arelowered. The stepped sections 57 e are linear or curved in shape so asto fit the right and left side edges of the upper face plate 71 andlower face plate 73 of the central frame section in plan. In view of theappearance of the joint welds between the central frame section 70 andthe legs 57, it is desirable to adjust the thickness of the upper faceplate 71 and lower face plate 73 such that the upper and lower faceplates 71, 73 are flush with the upper and lower faces, respectively, ofthe legs 57.

The vertical plate section which constitutes the front face of the basesection of the front leg section 57A is slightly set back in a backwarddirection to form the stepped section 57 g to which the right (left)side edge of the front face plate 75 of the central frame section isjoined. Similarly, the vertical plate section which constitutes the rearface of the base section of the rear leg section 57B is slightly setback in a forward direction to form the stepped section 57 m to whichthe right (left) side edge of the rear face plate 77 of the centralframe section is joined.

In the crawler frame of the fifth embodiment, the length S betweenflange sections 57 h, 57 i at the distal ends of the front leg section57A (the rear leg section 57B) and the base section 57 c of the frontleg section 57A (the rear leg section 57B) which base section isinserted into the side of the central frame section 70 is designed to beconstant as shown in FIG. 15. This means that the base section 57 c ofthe front leg section 57A is located at a position which is equivalentto the position of the base section 57 c of the rear leg section 57Bwith respect to a lateral direction. More specifically, the basesections 57 c of the front and rear leg sections 57A, 57B face eachother with such a proper spacing therebetween that they do not get intouch with the side supporting plates 74L, 74R which constitute the sidefaces of the central frame section 70.

Sixth Embodiment

FIG. 16 is a partly perspective view of a crawler frame according to asixth embodiment of the invention.

In the six embodiment, front and rear leg sections 67A, 67B respectivelyhave a base section 67 c of substantially rectangular cross-section. Avertical plate section 67 a is formed at the rear end of the basesection 67 c of the front leg section 67A, whereas a vertical platesection 67 b is formed at the front end of the base section 67 c of therear leg section 67B.

The vertical distance between the upper and lower faces of the rear endof the base section 67 c of the front leg section 67A is equal to thevertical distance between the upper and lower faces of the front end ofthe base section 67 c of the rear leg section 67B. The vertical platesection 67 a at the rear end of the base section 67 c of the front legsection 67A is chamfered off at its upper and lower corners such thatthe cut-away parts have a triangular cross-section. The vertical platesection 67 b at the front end of the base section 67 c of the rear legsection 67B is chamfered off at its corners such that the cut-away partshave a triangular cross-section. Then, the vertical plate section 67 aof the front leg section 67A is butted to the vertical plate section 67b of the rear leg section 67B, so that a V groove 69 is formed in eachbutted part 67 d. By forming welds in the V grooves 69, the front legsection 67A and the rear leg section 67B are integrated into abifurcated leg 67.

The base sections 67 c of the bifurcated legs 67 thus integrated, thebase sections having a substantially rectangular cross-section, areinserted into the right and left side faces of the central framesection, respectively. By virtue of a vertical plate section which has astepped section 67 g and constitutes the front face of the base sectionof the front leg section 67A; a vertical plate section which has astepped section 67 m and constitutes the rear face of the base sectionof the rear leg section 67B; the vertical plate section 67 a located atthe rear end of the base section of the front leg section 67A; and thevertical plate section 67 b located at the front end of the base sectionof the rear leg section 67B, the load imposed on the circular mount 15mounted on the top of the central frame section can be steadilysustained.

The upper and lower faces of the base sections of the front and rear legsections 67A, 67B are respectively provided with a stepped section 67 eso that the upper and lower faces of the base sections are lowered. Thestepped sections 67 e are linear or curved in shape so as to fit theright and left side edges of the upper face plate 71 and lower faceplate 73 of the central frame section in plan. In view of the appearanceof the joint welds between the central frame section and the legs 67, itis desirable to adjust the thickness of the upper face plate 71 andlower face plate 73 such that the upper and lower face plates 71, 73 areflush with the upper and lower faces, respectively, of the legs 67.

The vertical plate section which constitutes the front face of the basesection of the front leg section 67A is slightly set back in a backwarddirection to form the stepped section 67 g to which the right (left)side edge of the front face plate 75 of the central frame section isjoined. Similarly, the vertical plate section which constitutes the rearface of the base section of the rear leg section 67B is slightly setback in a forward direction to form the stepped section 67 m to whichthe right (left) side edge of the rear face plate 77 of the centralframe section is joined.

With the structure of the sixth embodiment, the entire circumferences ofthe base sections of the front and rear leg sections 67A, 67B aresecurely welded to the members of the central frame section, and as aresult, the integrally bifurcated legs 67 can be firmly secured to thecentral frame section.

In the sixth embodiment, the length S between flange sections at thedistal ends of the front leg section 67A (the rear leg section 67B) andthe base section 67 c of the front leg section 67A (the rear leg section67B) which base section is inserted into the side of the central framesection 70 is designed to be constant like the fifth embodiment shown inFIG. 15.

In the first and second embodiments, the base sections of the front andrear leg sections are respectively formed in the shape of an arc facehaving substantially the same curvature of the inner circumferentialsurface of the circular mount 15. Instead of this arrangement, thelength S between the distal ends of each bifurcated leg secured to theinner wall of the track frame and the base section of the leg may bemade constant. By virtue of the constant length S, it is possible toavoid use of a casting mold of complicated shape for the front and rearleg sections. In this case, the side supporting plates of the centralframe section are, of course, linear as shown in FIG. 12.

In the second and third embodiments, the side supporting plates aredisposed as the side faces of the central frame section for sustainingthe load of the upper structure imposed on the circle mount 15.Conversely, in the fifth and sixth embodiments, the side supportingplates 74L, 74R for sustaining the load of the upper structure imposedon the circular mount 15 are not necessarily provided for the right andleft side faces of the central frame section on the ground that thevertical walls (vertical plate sections) for sustaining the load of theupper structure imposed on the circular mount 15 are provided at severalpositions in the legs. Accordingly, the side supporting plates 74L, 74Rmay be omitted.

In the fifth and sixth embodiments, the lateral length S of the basesections of the front and rear leg sections is designed to be constant.Instead of this, the substantially rectangular base sections 57 c (67 c)of the front and rear leg sections may be curved so as to have the shapeof an arced face having substantially the same curvature of the innercircumferential surface of the circular mount 15 and the base sections57 c (67 c) may be inserted immediately under the circular innercircumferential surface of the circular mount 15.

1. A crawler frame for a construction machine, said crawler framecomprising: a center frame including: (i) a central frame section forsupporting a swing bearing and (ii) cast steel legs respectively locatedat right and left sides of the central frame section; and track framesdisposed at distal ends of the respective legs of the center frame;wherein each of the cast steel legs is bifurcated into front and rearseparately cast, cast steel leg sections; wherein a base section of eachsaid leg has a two-part structure; wherein a base section of the frontleg section of each said leg is securely welded to a base section of therear leg section of the leg; and wherein the base section of each saidleg is securely welded to the central frame section.
 2. The crawlerframe for a construction machine according to claim 1, wherein thecentral frame section includes an upper face plate, a lower face plate,a front face plate and a rear face plate which are welded to the basesection of each said leg, such that an entire circumference of the basesection of each said leg is welded to the central frame section.
 3. Thecrawler frame for a construction machine according to claim 1, whereinthe base section of the front leg section of each said leg comprises avertical plate section at a rear end thereof, and the base section ofthe rear leg section of each said leg comprises a vertical plate sectionat a front end thereof; and wherein the base sections of the front andrear leg sections are substantially rectangular in cross-section.
 4. Thecrawler frame for a construction machine according to claim 1, whereinthe central frame section comprises upper and lower face plates whichare joined to each said leg by J groove welds, and surfaces of the upperand lower face plates are flush with corresponding upper and lower facesof each said leg.
 5. The crawler frame for a construction machineaccording to claim 2, wherein the upper and lower face plates of thecentral frame section are joined to each said leg by J groove welds, andsurfaces of the upper and lower face plates are flush with correspondingupper and lower faces of each said leg.
 6. A crawler frame for aconstruction machine, said crawler frame comprising: a center frameincluding: (i) a central frame section for supporting a swing bearingand (ii) cast steel leas respectively located at right and left sides ofthe central frame section; and track frames disposed at distal ends ofthe respective legs of the center frame; wherein each of the cast steellegs is bifurcated into front and rear separately cast, cast steel legsections; and wherein each of the front and rear leg sections of eachsaid leg comprises a vertical wall which is formed from cast steel andwhich is integral with a base section of the respective one of the frontand rear leg sections.
 7. The crawler frame for a construction machineaccording to claim 6, wherein the legs are coupled to the central framesection such that the vertical walls are located substantiallyimmediately under a circular mount of the central frame section forsupporting the swing bearing.
 8. The crawler frame for a constructionmachine according to claim 6, wherein each said vertical wall comprisesa hole, which has a thickened lip, for having a hydraulic oil pipepassed therethrough.
 9. The crawler frame for a construction machineaccording to claim 6, wherein the central frame section comprises upperand lower face plates which are joined to each said leg by J groovewelds, and wherein surfaces of the upper and lower face plates are flushwith corresponding upper and lower faces of each said leg.
 10. Thecrawler frame for a construction machine according to claim 6, whereinthe central frame section comprises right and left side supportingplates, and base sections of the legs are inserted into and securelywelded to the central frame section so as to face a corresponding one ofthe side supporting plates.
 11. The crawler frame for a constructionmachine according to claim 10, wherein the side supporting plates arelocated substantially immediately under a circular mount of the centralframe section for supporting the swing bearing.
 12. The crawler framefor a construction machine according to claim 10, wherein each of theside supporting plates comprises a hole for having a hydraulic oil pipepassed therethrough, and a grommet is fit on a lip defining each saidhole.
 13. The crawler frame for a construction machine according toclaim 7, wherein the central frame section comprises upper and lowerface plates which are joined to each said leg by J groove welds, andsurfaces of the upper and lower face plates are flush with correspondingupper and lower faces of each said leg.